The present invention relates to a new and improved closure unit made of sheet metal or plating for sterilizable containers, to a new and improved method of manufacturing such a closure unit, and also relates to a container having a closure unit made of sheet metal or plating and obtained thereby.
Generally speaking, the closure unit of the present development is of the type comprising a holding or fastening ring which can be beaded or flanged over the associated container. The closure unit also comprises a ring area or region having a sealing surface for sealing thereto a membrane which has a tear-back tab and which covers a closure opening. The cut edge of the closure opening is folded or bent over into the sealing surface.
Closure units of the aforementioned type equipped with a membrane closure member which covers a container or closure opening and which can be peeled off or torn away from the holding or fastening ring of the closure unit, are used for containers, cans and the like for packing foodstuffs and industrial products.
Such type of closure unit is known, for example, from U.S. Pat. No. 4,253,584, granted Mar. 3, 1981, entitled "Ring and Closure for Cans". There is disclosed a cover ring suitable as a food can closure. The can for foodstuffs is made of lacquered aluminum or tin-plate. To protect the exposed cut edge laid bare by punching out the discharge or pour opening, the cut edge is bent over upwards and outwards into the sealing plane for the tear-back membrane. The unprotected cut edge is thus located beyond the filled material and cannot contaminate such filled material. Protection from corrosion by environmental factors, for instance during the filling of the container or by atmospheric moisture, is not in any way ensured.
To increase the resistance or strength of the sealing rim formed along the discharge or pour opening, a hollow curved portion is formed by the bent-over or turned-over cut edge. When the tear-back membrane is sealed onto the bent-over sealing surface, because of this hollow curved portion a sealing action with the required high sealing pressure only can be realized at the narrow ring area or region located beyond the hollow curved portion. The sealing surface and the membrane to be sealed thereto cannot be adequately and sufficiently pressed against each other in the region of the hollow curved portion in order to achieve a positive closure suitable for sterilization. This known ring and closure for cans have a further disadvantage in that when the ring is beaded or flanged over the can edge or rim, the membrane which in most cases is only a few hundredths of a millimeter thick and particularly the tear-back or pull-off tab folded back toward the center and onto the surface of the membrane can be damaged by the relative movement of the pressure plate of the beading or flanging machine at the beginning of the beading or flanging operation, such that the tearing back of the membrane is no longer thereafter ensured.
A further prior art construction of closure unit for a container is known from the published United Kingdom Patent Application No. 2,166,409 A, published May 8, 1986, entitled "Closures with Pull Tabs". Similar to the ring and closure disclosed in the aforementioned and discussed U.S. Pat. No. 4,253,584, this known closure comprises a hollow annular bead adjacent the container opening. When the composite membrane is bonded to the outer surface of the metallic closure, the hollow annular bead prevents the membrane from being sterilizingly sealed to the closure across the entire sealing surface thereof. To prevent damage to the pull tab by the pressure pad of the beading or flanging machine, the pull tab is double folded, thereby forming a double layer tab, such that at the most only one layer can be damaged by the beading or flanging machine while the layer lying therebelow can withstand the beading or flanging operation without damage.
Another construction of container cover of this type is known from Swiss Patent No. 563,284, granted May 15, 1975. There is described a metal foil membrane sealed onto the cut edge of the container opening. This cut edge is bent over upwards and outwards. Since the container opening is arranged in the central part of the closure cover and in spaced relationship to the flange or rim portion thereof which is to be bonded to the container, the tear-back tab is prevented from being damaged when the membrane or lid is sealed to the bent-over cut-edge portion of the metal foil membrane. However, the cut edge is not protected against corrosion.
Containers with a pour or discharge opening which does not span or extend across the entire cross-section of the container are not suitable for packing foodstuffs. In particular they are unsuitable for packing solid foodstuffs which cannot be poured out.
Still a further construction of closure cover is known from European Published Patent Application No. 0,090,957, published Oct. 12, 1983. There is described a closure cover for sterilizable containers, in which closure cover the cut edge of the pour opening is also bent over upwards and outwards into the plane of a hot sealing surface. This hot sealing surface is thus not only formed by the bent-over section, but moreover the peripheral region of the end surface located beyond the cut edge is shaped by deep drawing in that the surface of such peripheral region comes to lie in the plane of the bent-over cut edge. In this manner, there is achieved an enlargement of the hot sealing surface. This closure cover also has the disadvantage that the pull-off tab may be damaged by the pressure pad of the beading or flanging machine. Furthermore, the provision of the two-part hot sealing surface requires that both sides of the container cover ring must be provided with a sealing lacquer coating.